The present invention relates to calibrating and operating an electronic printer output device. It finds particular application in conjunction with calibrating an electronic printer output device using a tone reproduction curve (“TRC”) and will be described with particular reference thereto. Moreover, the present invention also relates to applying a TRC from a plurality of TRCs on a media basis within the printing of a single job. Furthermore, the present invention is directed to calibrating a plurality of TRCs based the calibration of a single media/halftone combination TRC based on a plurality of relationships between the single media/halftone combination TRC and the plurality of TRCs. It will be appreciated, however, that the present invention is also amenable to other like applications.
An electronic printer output device (“printing device”), also referred to as an image output terminal (“IOT”), is generally capable of producing color and standard black-and-white tones to produce images. Such a printing device may be a xerographic print engine, a thermal inkjet device, an acoustic ink print engine, piezo-electric print engine, etc. The printing device accepts color level specifications (typically for each of four (4) colors including cyan, magenta, yellow, and black (“CMYK”) or more colors in a high fidelity print engine) as input and produces corresponding color areas on a printed page. This production of the corresponding color areas is often performed by a screening operation (also referred to as a halftoning operation) wherein a fine pattern of color spots is printed. The spots are grouped together to form dots. The dots appear as varying color tones in accordance with the number of color spots used when viewed from a distance. The allowed color level specifications for each of the CMYK color spaces generally vary over some finite range such as 0 to 1 or 0 to 255. The colors at the extremes (i.e., 0 and 1 or 0 and 255) are white and saturated CMYK colors, and numbers between the extremes yield intermediate color tones. However, while the color level specifications may vary linearly, a linear change in color levels typically does not result. Instead, there may be a particular threshold before lighter toned colors become visible. Similarly, darker toned colors may prematurely appear saturated.
A printing device is designed to operate under certain conditions. More specifically, a printing device is calibrated to operate at an ideal set-point, for producing relatively consistent output when used within an environment having specific desired operating parameters such as, for example, a specific humidity, temperature, and dust count, etc. If one or more of the operating parameters deviate from the desired operating parameters, the printing device may drift away from the ideal set point. To some extent, the drift can be corrected with controlled feedback mechanisms within the printing device itself, but typically the printing device requires further corrective action to maintain color consistency in the printed output. The present invention relates to this corrective action.
One way to compensate for the drift within the printing device includes modifying data within a printing device output device controller (“controller”). The controller converts image data (e.g., PostScript® data, PCL data, or PDF data) into raster data, which is transmitted to the printing device where it is printed. More specifically, a TRC modifies the raster data before it is sent to the printing device. A TRC is an electronic map (a graphic representation of the relationship of input v. output) that converts an input image value to a desired image out value. During calibration of a black and white print engine, a test suite of grey patches may be printed out from known image values (grey levels) and the resulting test patches are measured to determine their actual printed image value. A perfect black and white printer would have a TRC that is a linear relationship between the input and the output. In calibration, the actual output of the printing device is measured and a relationship between the input and output is established. The calibration TRC is generated by determining the difference between the ideal and the actual, thereby allowing the TRC to modify the raster data to correct for drift. The resulting image produced by the printing device using the corrected raster data is printed correctly (i.e., as if the printing device had not drifted from the ideal set point and the data was not corrected by the TRC). Both linear and nonlinear drifts within a printing device may be compensated by using calibration TRCs.
In order to accurately produce a desired color level, the correspondence between the color level specification of the printing device and the actual color level produced by the printing device must be known. The TRC provides an illustration of the corresponding relationship.
In order to determine the TRC, a sample set of color level specifications is printed and a device such as a colorimeter or spectrophotometer measures the color levels actually produced by the printing device. The measured points are then used as raw data to calculate a set of curves. The set of curves produced is referred to as the calibration TRCs, or simply as TRCs. Various methods exist for creating TRCs. One such method is set forth in U.S. Pat. No. 5,696,889, which is hereby incorporated by reference.
Although TRCs can be created and applied to correct for drifts in a printing device output, there is currently no method or system that permits a user to specify a specific TRC based on media-type. Moreover, there is currently no method or system that permits a user to specify a specific TRC based on a media/halftone combination. Thirdly, there is currently no method or system that permits a user to specify a specific TRC on a page basis within a single print job. Lastly, there is currently no method or system that permits a user to readjust a plurality of TRCs based the calibration of a single TRC using a reference media and a reference halftone or plurality of reference halftones.
The present invention provides a new and improved apparatus and method that overcomes the above-referenced problems and others.